1. Field of the Invention
The present invention relates to a method of manufacturing a foam-molded product having a surface condition that is free of any silver mark or flash mark and is as good as that of a non-foamed product and hence does not require any skin material.
2. Description of the Related Art
In recent years, every industrial field including the vehicle parts industry demands that all the articles to be used therein be products that are lighter in weight than and comparable in price and performance to conventional ones. Foam molding is one of techniques that can meet this demand. Among vehicle interior trim parts such as a door trim, front trim and rear trim, for instance, there are products of the type which is affixed with a fabric skin material of which base resin is low-foamed. Though such a product is light-weight and has a good surface appearance by virtue of the skin material, the skin material may wrinkle when affixed to products of some shapes, namely rough-surface products. For this reason, applications of such products are limited.
Japanese Examined Patent Gazette No. HEI 7-77739 has proposed a method of providing a molded product that is wholly formed of a foamed material without using any skin material. This art uses molds having fitting portions that are slidable relative to each other, adjusts the cavity clearance to 1 mm or less, and injects a polypropylene resin containing a foaming agent into the molds while enlarging the cavity clearance. During the injection the cavity clearance is adjusted so that the pressure of the resin within the cavity assumes 5 to 100 kg/cm2. With this state being kept, the surface of the resin in contact with the cavity surface is allowed to form a skin layer. Once the formation of the skin layer has been completed, one of the molds is moved to a predetermined position to enlarge the cavity clearance so that foaming of an uncured portion inside the resin thus filled is caused. With this state being kept, the resin is solidified by cooling and then removed from the molds, thus giving a foamed product.
Though this method is capable of molding a foamed product without using a skin material as described above, it does not necessarily yield a molded product with a good appearance because when the polypropylene resin previously mixed with the foaming agent is injected from an injection cylinder of which the inside is held at a high pressure into the mold held at atmospheric pressure, the pressure on the filling resin rapidly lowers and, hence, part of the foaming agent present adjacent the surface of the resin foams during the filling of the resin, with the result that the foam thus produced is extended or stretched by the flow of the resin thereby forming a flash mark, silver mark or a like mark on the surface of the foamed molded product in some cases.
Further, in the case of the manufacture of a molded product having a thin wall portion, it is very difficult to fill an adequate amount of the polypropylene resin containing the foaming agent into a portion of the cavity defining such a thin wall portion because the foaming agent contained in the resin filled in the cavity of the mold kept at atmospheric pressure is constantly urged to foam thereby increasing the resistance of the resin against such a thin wall defining portion of the cavity. Thus, it is possible that the resin fills the thin wall defining portion of the cavity insufficiently thereby causing incomplete formation of a thin wall portion.